Well, I just did it by mistake!
On my Pokemon yellow, I've been fooling around with the Ditto/Mew trick and seeing what pokemon I would get. I went up to a trainer in Viridian Forest, used escape rope before he "saw" me, fought another trainer, went back to Viridian forest, and found various pokemon through various trials depending on the last pokemon I saw. I caught a Nidoking this way.
So, I was trying that out on Mt. Moon. I saved my game right before I went inside, went up to a Rocket and used escape rope before he "saw" me, and fought other trainers to see what pokemon would appear when I went back to where the Rocket was (one was an Exeggcute). I didn't catch any of them, though, and I would turn off my game so I could repeat the experiment again.
So today I turned on my game and I was outside Mt. Moon, where I had saved last. I decided to level up my Pikachu a bit, so I let it fight some wild pokemon in the grass and in Mt. Moon itself. It leveled up from 14 to 15, and it learned Double Team.
I saved my game right outside Mt. Moon like I always did, and I turned it off. When I turned my game back on, I was no longer outside of Mt. Moon, but in Glitch City!
I can walk on a 1x2 patch of ground and a 2x2 patch of grass. I can see a portion of a "Mart", and most of the glitch region is halves of houses. When I talk to my Pikachu he seems content and unaware of the sudden change of environment. I can actually find wild pokemon in the grass, and they seem perfectly normal. There are Rattatas, Mankeys, Spearows, etc, and are between levels 7-11. I even caught one of the Spearows (I only had 3 pokemon in my party), and he is not glitched either. Besides the fact that I'm stuck in Glitch City, nothing else is glitched. None of the pokemon know Fly, Dig or Teleport, and my escape rope won't work, so I'm literally stuck!
If you would like to see images, all I have are crappy pictures from my camera, but I would be happy to upload them if anyone is interested. You can see in my pokedex that I only have the Boulder Badge and I have less than 3 hours on the clock.
EDIT: I just realized: I could have all my pokemon get killed by the wild pokemon in the grass, and I could get out! I did that, and I returned to that pokemon center outside Mt. Moon, and everything seems ungliched afterward.